Volume 78, Issue 6, Pages 3138-3149 (June 2000) Influence of Ionic Strength on the Actomyosin Reaction Steps in Contracting Skeletal Muscle Fibers  Hiroyuki Iwamoto  Biophysical Journal  Volume 78, Issue 6, Pages 3138-3149 (June 2000) DOI: 10.1016/S0006-3495(00)76850-0 Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 1 Effect of ionic strength (IS) on the contraction of skinned rabbit psoas fibers activated by the photolysis of caged Ca2+. (A) time course of rise of isometric tension after photolysis. All records were normalized to the final level of tension reached after photolysis at normal IS (IS=200mM, gray line; three traces are superimposed). The black traces were obtained at IS values of 120, 360, and 520mM from above. Three sets of paired records (at normal and high or low IS values) from three different specimens are shown. (B) Summary of effects of ionic strength on isometric tension, stiffness, and the rate constant for the rise of tension (kdev) of the fibers activated by caged Ca2+ photolysis. All of the values were normalized to the values at normal IS (IS=200mM). The columns denoted by 0, 1, 2, and 3 represent the IS values of 120, 200, 360, and 520mM, respectively. Mean±SD (n=4–6). Asterisks mark levels of statistical significance of the difference from control values: *p<0.05; **p<0.01. The absolute value of kdev at IS=200mM is 7.2s−1. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 2 Effect of IS on the contraction of skinned rabbit psoas fibers activated by solution exchange. (A) Lightly loaded shortening and subsequent redevelopment of isometric tension. The traces above are length records (scale bar, 0.5% fiber length). The fibers were made to shorten under a constant load of 5% isometric tension for 60ms and then were held isometric. Tension records were normalized to the level of tension immediately before shortening at normal IS (IS=200mM). Gray lines, records obtained at normal IS; black lines, records for higher IS values. In tension records, the traces are on the order of 360 and 520mM from above. In length records, the traces are on the order of 360 and 520mM from below. All traces were recorded from the same specimen. (B and C) Summary of effects of ionic strength on the static and dynamic parameters of fibers activated by solution exchange. (B) From the left, prerelease levels of isometric tension and stiffness, kdev, recorded after lightly loaded shortening (LL, records shown in A) and after no-load shortening (NL) (the rate of rise of tension was measured after the fiber was made slack by imposing a large quick release; records not shown) and the velocity of lightly loaded shortening (records shown in A). (C) Data obtained in the presence of 20mM Pi. From the left, prerelease levels of isometric tension and stiffness, and kdev recorded after lightly loaded shortening (LL) (records not shown). All of the values were normalized to the values at normal IS (IS=200mM). The columns denoted by 1, 2, and 3 represent the IS values of 200, 360, and 520mM, respectively. Mean±SD (n=5–8). Asterisks mark levels of statistical significance of the difference from control values: *p<0.05; **p<0.01. The absolute values of kdev at IS=200mM are 8.3 and 14.2s−1, respectively, in the absence and presence of 20mM Pi. The shortening velocity in the absence of Pi is 0.33×fiber length·s−1. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 3 The relation between isometric tension and stiffness of fibers activated by caged Ca2+ photolysis. The values were normalized to those of fibers fully activated at IS=200mM. Black squares, fully activated at IS=120mM; black circles, fully activated at IS=360mM; black triangles, fully activated at IS=520mM; gray circles, activated to various levels at normal IS (200mM) by reduction of the energy of the laser flash. The line (single exponential association) is drawn only to assist the eye. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 4 Tension responses to ramp stretches of various amplitudes (duration, 10ms; amplitude, 0.3–1.2% fiber length in 0.3% steps) recorded at the plateau of contraction initiated by solution exchange. (A and B) Traces of tension (below) and length (above) recorded at an isometric plateau at IS=360mM in the absence (A) and presence (B) of 20mM Pi. Four traces are superposed. Tension records were normalized to the prestretch level (Po). The dotted line represents the zero-tension level. Note that the tension responses to a 0.3% stretch are not different, but for larger stretches the tension responses become disproportionately greater in the presence of Pi (B). (C) The relation between stretch amplitude and the magnitude of tension response, defined as the difference between the prerelease level of tension and the peak tension attained at the point where the stretch stopped. ○, IS=200mM,−Pi; ●, IS=360mM,−Pi; ■, IS=360mM,+20mM Pi. Note the increased slope of the curve in the presence of Pi at stretch amplitudes of ∼0.3% fiber length (Lo) and greater. Mean±SD (n=6, n=2 for IS=200mM). For more results for IS=200mM see Iwamoto (1995a,b). Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 5 Tension responses to ramp stretches of various amplitudes (duration, 10ms; amplitude, 0.3–1.2% fiber length in 0.3% steps) recorded in the rising phase or at the plateau of contraction initiated by caged Ca2+ photolysis. (A and B) Tension and length traces recorded at IS=200mM. (C and D) Tension traces recorded at IS=360mM. Traces were obtained 150ms (A and C, rising phase; ∼30% of full isometric tension has developed) and 3s (B and D, isometric plateau) after flash photolysis of caged Ca2+. The tension traces were normalized to the level immediately before stretch. Note the disproportionately enhanced tension response to stretches greater than 0.3% of fiber length (Lo) in the rising phase at both IS values (A and C). (E and F) The relation between stretch amplitude and the magnitude of tension response to stretches applied 150ms (E) and 3s (F) after photolysis. ○, IS=200mM; ●, IS=360mM. The scale of the ordinate to the left applies to IS=200mM; that to the right applies to IS=360mM. The line connecting the data points at 0 and 0.15% Lo and the line connecting those at 0.45 and 0.6% Lo are extended with dotted lines to stress the difference in the shape of curves between E and F. Note that the curves are nonlinear around 0.3% in E and more linear in F, regardless of IS. Mean±SD (n=6). Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 6 Measurement of the rate constant for the decay of stiffness (kdecay) after a quick release. Most of the decay occurs during the plateau period (phase 3; see Huxley and Simmons, 1971) of tension transient. (A) Tension response to a quick release (0.8% fiber length, complete in 1ms, trace above; scale bar, 0.5% fiber length). (B) Accompanying changes of stiffness (squares) measured with sinusoidal oscillation (0.2% fiber length in peak-to-peak amplitude, 1kHz). Records were normalized to their prerelease levels at normal IS (gray line and squares). Solid records are obtained at 520mM. Traces were recorded from the same specimen. (C) Summary of the IS effect on kdecay, normalized to the value at IS=200mM. For the meaning of numbers and asterisks by the columns see Fig. 1. Mean±SD (n=6). The absolute value for kdecay at IS=200mM is 160s−1. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 7 Effect of IS on rigor stiffness. (A) Tension response (traces below) of rigor fibers to a series of stretches of equal amplitude (traces above; scale bar, 0.5% fiber length). The tension increment with a step (or stiffness (squares) in arbitrary units) increased with steps but tended to saturate. The gray and black records are for normal and 520mM IS, respectively. All data were recorded from the same specimen. (B) Summary of the IS effect on rigor stiffness. The highest values of stiffness recorded in a single series of stretches were normalized to those at IS=200mM. For the meaning of the numbers and asterisks by the columns, see Fig. 1. Mean±SD (n=6). Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 8 Actomyosin reaction scheme proposed to explain the present results. The scheme has three force-generating A·M·ADP substates (denoted by 1 and 2), the latter bearing higher force. Three rate constants (k+2, k−4′, and k−5, denoted by §) are assumed to be IS-dependent (k+2 is decreased, and k−4′ and k−5 are increased by increasing IS). The release of ADP and the dissociation of A·M·ATP are assumed to be irreversible. Following the low-force A·M·ADP·Pi (denoted by L), a force-generating A·M·ADP·Pi intermediate (denoted by F) is incorporated in the light of a two-step Pi release process (Dantzig et al., 1992; Kawai and Halvorson, 1991). This intermediate, assumed to support the same amount of tension as A·M·ADP (1), does not exist in a significant amount in the absence of Pi. (A) The whole scheme of ATPase reaction in which all intermediates are aligned in series. The right end of the scheme continues to the left end. (B) A part of the scheme in A, but with a branch pathway allowing the A·M·ADP(1) to release ADP (gray arrow and primed rate constant). (C) A part of the scheme in A, but with a second branch pathway allowing the direct formation of A·M·ADP(2) from A·M·ADP·Pi(F) (gray arrow and primed rate constant) along with the branch pathway in B. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions

Figure 9 Simulated isometric tension, stiffness, ATPase rate, and kdev based on the schemes in Fig. 8. (A and B) Simulation based on the scheme in Fig. 8 A, with all of the intermediates in series. (C and D) Simulation based on the scheme in Fig. 8 B, in which one of the branch pathways (ADP release from A·M·ADP(1)) is allowed. The result of the simulation based on the scheme in Fig. 8 C, in which both branch pathways are allowed, is almost identical to that in C and D and is therefore not shown. In each set of four columns (two white, two shaded), the two columns (one white, one shaded) on the left side represent the values at normal IS, and the two on the right represent the values at high IS (intended to reproduce the results at 520mM). The white and hatched columns represent, respectively, values in the absence and presence of 20mM Pi. In A and C the values were normalized to the value at normal IS in the absence of Pi. In B and D the same data as in A and C were normalized in a different manner to stress the effect of Pi on IS sensitivity: the values were normalized to the value at normal IS at respective concentrations of Pi. The force was calculated by assuming that the force produced by a myosin cross-bridge in the A·M·ADP·Pi(F) or A·M·ADP(1) state is 20% of that produced by a myosin head in the A·M·ADP(2) state. The myosin head in the A·M or A·M·ATP state is assumed to produce the same amount of force and stiffness as that in the A·M·ADP(2) state, although such intermediates are virtually nonexistent with the present set of rate constants. The stiffness was calculated by assuming that all of the force-generating cross-bridges make an equal contribution to the stiffness, while the A·M·ADP·Pi(L) cross-bridges make a 50% contribution. The dissociated forms of intermediates were assumed to bear no stiffness. The time course of rise and the final amount of tension or stiffness were calculated by numerically integrating a set of differential equations derived from the scheme in Fig. 8 and rate constants listed below, with a unit time segment of 40μs. The initial condition was that all of the intermediates were in the form of M·ADP·Pi. The value of kdev was calculated as the inverse of the time required for the isometric tension to reach its half-maximum. The values of the rate constants used in the simulations are (in s−1) k+1=50; k−1=10; k+2=10 (normal IS) or 3.85 (high IS); k−2=10; k+3=100; k−3=200; k+4=2000; k−4=100,000×[Pi] in M (in A–D); k+5=100; k−5=100 (normal IS) or 676 (high IS); k+6=6 (in A and B) or 3 (in C and D); k+6′=3; k+7=1000; k−7=1000; k+8=1000. For the calculation for the scheme in Fig. 8 C (not shown because the results are almost identical to those in C and D), the rate constants used are k+4′=1000; k−4′=50,000×[Pi] (normal IS) or 338,000×[Pi] (high IS); k+4=1000; k−4=50,000×[Pi]. Note the large effect of IS on ATPase in A and B. The addition of 20mM Pi increases the sensitivity of tension to IS. Biophysical Journal 2000 78, 3138-3149DOI: (10.1016/S0006-3495(00)76850-0) Copyright © 2000 The Biophysical Society Terms and Conditions